Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR CONTROLLING OXYGEN WHEN SEPARATING
MINERALS FROM A SLURRY
The invention relates to a method for controlling the partial pressure of
oxygen when mutually separating minerals from a slurry containing valuable
s minerals in various different sub-steps of a process. The partial pressure
of
oxygen is controlled by recirculating the gas conducted into the process or
into various sub-steps of the process.
After crushing and grinding, the various different minerals contained in ore
to are mutually separated, as selectively as possible, in different sub-steps
of
the process, such as flotation, precipitation and filtering, from the slurry
to be
treated. From the point of view of the process, it is important that the rate
of
oxidation on the surface of the different minerals, i.e. the electrochemical
potential of the minerals, is controlled by means of the oxygen contained in
is the air and by chemicals, so that the minerals to be flotated are made
hydrophobic on the surfaces, and respectively the minerals to be settled are
made hydrophilic. An overoxidation of the minerals results in nonselectivity
and weakened recovery, as well as in increased chemical expenses.
2o It is well known in the prior art that the oxygen to be conducted it1 the
various
sub-steps of the process is the largest factor affecting the oxidation of the
mineral surfaces. For example, in flotation there is normally used air as the
flotation gas, and thus the quantity of the dissolved oxygen is more or less
saturated, when the quantity of oxygen is calculated on the basis of the 21
2s contained by air. It is also well-known in the prior art that one of the
most
important reasons for an unsatisfactory operation of the sub-processes
following flotation, i.e. precipitation and filtering, is the lack of control
of the
oxidation-reduction conditions in the slurry under treatment.
so In the patent US 6,044,978, there is described a method for recovering
copper and nickel, as well as platinum group metals (PGM) from sulfidic ores
containing said metals. In said method, reagent conditioning is carried out in
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conventional fashion, but further conditioning of the slurry is carried out in
the
presence of a non-oxidizing gas, such as nitrogen. The flotation steps are
performed normally by air or at least by a gas with an oxygen content that is
higher than that of a gas used in the conditioning step preceding flotation.
s According to the US patent 6,044,978, the use of a non-oxidizing gas
improves the separation of sulfide minerals from the rest of the ore.
However, the use of nitrogen or other non-oxidizing gases such as carbon
dioxide, methane, ethane, propane or sulfur dioxide is not necessarily
to economic. For example the use of nitrogen or hydrocarbons requires that a
corresponding plant is built in connection with the concentrator.
The object of the invention is to eliminate drawbacks of the prior art and to
achieve an improved method for mutually separating minerals, in order to
i's control the process from a slurry containing valuable minerals, by
adjusting
the partial pressure of oxygen of the gas fed in the process, so that the
gases fed in the separate steps of the process are recirculated. The essential
novel features of the invention are enlisted in the appended claims.
2o According to the present invention, the partial pressure of oxygen is
controlled in order to mutually separate minerals in the various steps of
treating a slurry containing valuable minerals by recirculating the gas
conducted to the subsequent steps, i.e. flotation, precipitation and
filtering.
Part of the oxygen is consumed in the oxidation reactions of the mineral
2s surfaces, and when the gas exhausted from the treatment is recirculated,
the
nitrogen content of the gas is increased, and the potential of the mineral
surfaces can be adjusted by means of the recirculation gas. In a
concentration process, for instance sulfur compounds and chemicals
consume oxygen, and the oxygen content of the atmosphere is decreased,
so when the same gas is reused. The additional oxygen needed in the process
is fed for instance in the form of air, oxygen or oxygen enriched gas,
according to the oxidation requirements of the minerals in question. The
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additional oxygen can also be fed so that part of the additional oxygen is
ozone or chlorine.
In a process according to the method of the invention, essentially all
suitable
s steps, such as grinding, pumping, conditioning and flotation steps as well
as
both precipitation and filtering are closed in a recirculation gas atmosphere.
In this way, there is avoided an excessive oxidation of the minerals in all
process steps. For example pump wells, when they are open, function as
mineral slurry oxidizers, and therefore also pump wells are integrated in the
to closed gas circulation. A closed gas circulation means that the equipment
belonging in the process is provided with the necessary structures, such as
lid structures, in order to ensure the recovery of gas. In addition, a process
applying the method according to the invention is provided with equipment
needed for gas transfer and recirculation, such as recirculation pipework, at
is least one fan and a storage tank. In order to define the correct oxygen
content of the recirculation gas, the method is observed for instance by
measuring the share of oxygen in the recirculation gas, or by observing the
potential of the ore slurry. In order to observe the slurry to be treated in
the
process, for measuring the state of the solid or pulverous surfaces of tile
2o mineral electrodes in tile process slurry, there are applied potential
measurements, impedance measurements, ultrasound initiated or potential
pulse initiated zeta potential measurements, either separately or together
with measurements of the contents of reagents present in the process or
added in the process. Among the surfaces to be measured, there can also
2s be the specific surfiaces of the equipment or of the equipment parts used
in
the process.
According to the invention, the overoxidation or overreduction of the minerals
contained in the slurry can be prevented by a method that is as expedient
3o and cost-effective as possible. Most chemical reactions that take place in
the
different process steps are practically irreversible. Once the change has
taken place, there is no technical method for returning tl~i~e~inerals to the
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optimal state. In order to avoid overoxidation or overreduction, the feeding
of
secondary gas according to the invention is divided according to the different
process steps, so that to each process step, there is fed either the same
secondary gas, or the partial pressure of oxygen in the secondary gas is
s changed, when necessary, between the gas additions fed in the different
process steps. Thus, in case it is necessary on the basis of the performed
measurements, recirculation gas can be used in at least one process step,
the content of said oxidizing gas being lower than the oxygen content of air.
In that case for instance hydro sulphide or sdulphur dioxide can be used as
to the reducing gas. Likewise, when necessary, at least in one process step
there can be used recirculation gas, the content of said oxidizing gas being
higher than the oxygen content of air.
The closed gas circulation according to the invention results in that the
whole
is process is made to work in an economically advantageous way with
oxidation potentials lower than in the prior art. Thus it is possible to
achieve a
better selectivity in the separation of minerals, and a completely new level
of
operation is achieved as regards the mineral recovery-content graph. At the
same time, reagent expenses are decreased, and the productivity of the
2o process equipment grows. ii~loreover, the suction and underpressure
naturally created by the rotation of the propellers or other agitation
equipment employed in the treatment of valuable minerals can be made use
of. Likewise, in order to improve the efficiency of the gas recirculation, in
adjusting the contents of the oxidizing gases, there can advantageously be
2s utilized the natural impoverishment of the oxidizing gases taking place in
the
process.
Because the whole process of treating the slurry containing valuable metals
is carried out in a closed gas circulation, this means for instance that the
so oxygen content in grinding is on a lower level than in when operating in a
normal air atmosphere. As a consequence, the corrosion of the lining
materials of the mill used in the grinding process is slowed down, and the
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consumption of the balls and rods used in the mill is decreased.
Advantageous effects of the method can also be observed in the other steps
of the process.
s In flotation, among the many significant factors, the structure of the froth
has
been and still is one of the most essential. In the final estimation froth
structure, if nothing else, reveals the problems possibly caused by the
flotation gas, depending on the raw material to be treated, on the chemicals
etc. When recirculation gases are in the flotation process used according to
to the invention, the froth structure can be optimized remarkably more
efficiently. The optimization of the froth structure can be carried out from
step
to step in order to correspond to the separation selectivity of the minerals
and to the requirements set for froth structure by the transfer of the
minerals
faking place in the froth, such as mechanical strength. Thus also heavy
is minerals, such as precious metals and platinum group minerals can be kept,
for instance by loose bonds, bound to mutually connected particles, i.e.
flocks, or to flocks, i.e. misels, formed by different molecules, and at the
same time the specific weight is reduced by means of the hydrocarbons.
20 lfilhen applying the method according to the invention, stronger mixtures
can
be used in the process, adjusted according to the conditions at hand. For the
process, this in turn means shorter delays, higher capacities and lower
expenses, without the drawback that the recoveries to a desired phase
should become smaller. The method according to the invention is particularly
2s suited to the flotation, precipitation and filtering as well as other
connected
processing of ores and raw materials containing copper, nickel, zinc, cobalt,
lead, silver, platinum and palladium.
The invention is described in the following example with respect to flotation.
Example
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The ore to be treated in the process contained both copper sulfides, such as
chalcopyrite CuFeS2, bornite Cu5FeS4, different CuXS phases and pyrite
FeS2.
The ore was ground together with alloyed metal grinding bodies to the
fineness 89.3 % below 74 micrometers, in a nitrogen atmosphere that
contained 11 % oxygen. The flotation was performed according to the
invention in a closed gas circulation created around the grinding and
flotation
equipment by using conventional colhectors, i.e. xanthate and dithio
to phosphate by nitrogen containing 2% oxygen at the pH 10.8, by applying an
addition of Ca(~H)2.
As regards the electrochemical potentials of chalcopyrite CuFeS2 and pyrite
FeS~, the conditions were optimized, so that E~"F~s2 was within the range -
is 160 mV vs AgCI/Ag and EFes~ - 75 mV vs AgCI/Ag. The administration of the
flotation reagents was optimized by maximizing the hydrophobicity
differences by means of impedance analysis between chalcopyrite and
pyrite.
2o In flotation, the obtained FeS2 content for the copper preconcentrate was
8.4%, and the obtained silicate content was 4.7%. By repeating the
preconcentrate in the above described conditions, the copper recovery of the
final copper concentrate, when measured from the ore, was 93.5 %, and the
FeS2 content of the copper concentrate was 4.4 % and the silicate content
25 was 2.2 %.
When a corresponding experiment, with the same grinding, was according to
the prior art performed in an air atmosphere, without the closed gas
circulation created around the flotation equipment, the flotation gas was air,
3o and the CuFeS2 potential was optimized to the value - 160 mV vs AgCI/Ag,
the FeS2 content in the copper preconcentrate was 15.6 % and the silicate
content was 6.8 %. In the final copper concentrate, repeated by using air, the
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FeS2 content was 12.5 %, the silicate content was 4.7 % and the copper
recovery was 88.6 %. Let us point out that with respect to selectivity, the
FeS2 potential was increased too high.
s When comparing in the example the values obtained by a method according
to the invention with the values obtained by a method according to the prior
art, it can be maintained that the copper recovery was improved by 4.9%, the
silicate content was decreased more than 50%, and the pyrite content was
decreased by 65%. Now the obtained copper concentrate was purer and
to thus easier to process further.
